1) Field of the Invention
The present invention relates to a support sheet for a photographic printing sheet. More particularly, the present invention relates to a support sheet for a photographic printing sheet having a superior resistance to scratching and curling and an enhanced ability to be sharply cut.
2) Description of the Related Arts
A water-resistant resin-coated paper sheet consisting essentially of a substrate paper sheet coated on both surfaces thereof with a polyolefin resin is now used as a support sheet for a photographic printing sheet, in place of the former conventional baryta paper sheet, to cope with the current high speed developing treatment required for photographs.
In general, the support sheet for a photographic printing sheet must satisfy the requirements for a superior dimensional stability, resistance to humidity, adhesive activity and shielding activity, and must not affect the photographic emulsion layer formed thereon. Further, the support sheet must have a high resistance to curling and scratching and must be able to be sharply cut with a cutter or edge.
Usually, the support sheet for a photographic printing sheet is provided with a clear coating layer formed on one surface of a substrate sheet and composed of a clear polyolefin resin, and a opaque coating layer formed on the opposite surface of the substrate sheet and composed of an opaque mixture of an polyolefin resin with a pigment, for example, fine titanium dioxide particles. A photographic emulsion layer is formed on the opaque polyolefin resin coating layer (front surface layer).
Japanese Examined Patent Publication (Kokoku) No. 48-9963 (1973) discloses a support sheet for a photographic printing sheet, of the above-mentioned type. In this support sheet, a front surface coating layer on which the photographic emulsion layer is to be formed is provided by a high pressure-processed low density polyethylene resin (HP-LDPE), and a back surface coating layer is formed from a middle pressure-processed or low pressure-processed high density polyethylene resin alone or a mixture thereof with an HP-LDPE resin. In this type of support sheet, the density of the polyethylene resin in the front surface coating layer, on which the photographic emulsion layer is to be formed, in usually lower than that in the back surface coating layer, to prevent a curling of the photographic printing sheet made from the support sheet.
Also, in a photograph processing laboratory, images are printed on the photographic printing sheet through a negative film, developed to prepare a print with a positive image, and the resultant developed print sheet is cut to predetermined dimensions by a cutter or the like. When cut, the support sheet having the polyolefin resin coating layers or the photographic printing paper sheet or print sheet made from the above-mentioned support sheet is subjected to a shearing force created between upper and lower edges of the cutter. In this cutting procedure, the polyolefin resin coating layer is often stretched by the cutting force, and thus a sharply cut face thereof is not obtained. The support sheet, photographic printing sheet or print sheet having the above-mentioned stretched cutting face has an unsatisfactory appearance, and thus has a low commercial value.
Also, when the HP-LDPE resin is used, the resultant coating layer of the support sheet for a photographic printing sheet has a poor resistance to scratching. Namely, when the front surface coating layer comprising the HP-LDPE resin is brought into direct contact with guide rollers in a coating machine for coating thereon the photographic emulsion, or the back surface coating layer comprising the same polyethylene resin as mentioned above is brought into contact with a winding roller, those coating layer surfaces are often scratched, and the scratches formed on the front or back surface coating layer remain even after the photographic emulsion layer is formed on the front surface coating layer and the resultant photographic printing paper sheet is printed and developed, and therefore, the commercial value of the resultant print is lowered.
Recently, an attempt was made to enhance the resistance of the photographic printing paper to scratching by using a linear low density polyethylene resin (L-LDPE), as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 2-73246 (1990). This type of photographic printing paper sheet does have an enhanced resistance to scratching and curling, but the photographic printing paper sheet cannot be satisfactorily sharply cut with a cutter or edge, and thus is not commercially viable.
Japanese Unexamined Patent Publication (Kokai) No. 63-301945 (1988) discloses a support sheet in which an L-LDPE resin is used to enhance the dispersion of titanium dioxide particles in the coating layer. The inventors of the present invention repeated this process and found that the enhancing effect of the L-LDPE resin on the titanium dioxide particles was not recognized and the resultant photographic printing paper sheet could not be satisfactorily sharply cut with a cutter. Also, it was recognized that any improvement in the dispersion is greatly influenced by the melt-flowing property of a resin to be used to dilute a master batch, as described in Japanese Unexamined Patent Publication (Kokai) No. 62-150248 (1987).